Institute of Electrical and Electronics Engineers (IEEE) 802.11 TGah defines the concept of synch frame for saving power consumption of stations. When requested by a station (STA), an access point (AP) sends a synchronization (synch) frame at the slot boundary or the target wake time of the STA, if the channel is idle, to help the STA quickly synch to the medium. This is optional to the AP and STA. It is recommended that the AP sends a null data packet (NDP) clear-to-send (CTS) frame as a synch frame. The use of a synch frame is very useful because STAs do not need to wait and listen to the wireless medium for ProbeDelay time, which consumes quite a lot of power.
FIG. 1 illustrates a short synch frame example 100. The AP transmits a beacon frame 102 that the STA receives in the awake state. If the AP determines that the channel is idle at the slot boundary 104, the AP transmits a short synch frame 106 at the slot boundary 104 to help a station to quickly synch to the medium. The STA wakes up at the slot boundary 104 and waits for channel synch. If the STA receives the short synch frame 106 from the AP at the slot boundary 104, the STA starts to access the channel right after the reception of the short synch frame 106 and transmits data 108. The channel access follows the enhanced multimedia distributed control access (EDCA) rules. The STA does not need to sense the channel for a long ProbeDelay time to synch to the medium. The AP may transmit an acknowledgement (ACK) frame 110 to the STA after receiving the data 108.